This study tests a new method for determining glomerular filtration rate (GFR). The accurate determination of GFR is currently a costly and time- consuming procedure. Dr. Rocco has recently developed a method of measuring radiocontrast material using capillary electrophoresis. In this study, he injects 1,750 mg. of iohexol intravenously into patients and measures the concentration of this material at several time points after the injection. He then uses a variety of kinetic models to determine GFR.